formatters: color output on ANSI terminals.

* specl-rockspec.lua (dependencies): Add ansicolor.
* specl/std.lua (prog.usage): Describe new --color option.
(optarg): Factor out extraction of argument options after '='.
(process_args): Use it for --formatter processing.
Add support for --color option.
* specl/specl.in (opts): Make global, for visibility in color.lua.
* specl/color.lua (color): New file.  Enable or disable colored
output according to opts.color setting.
* specl/specl.mk (nobase_dist_lua_DATA): Add color.lua.
* specs/specl_spec.yaml: Add `--color=no` to specl invocations for
saner output comparison checks.
* specl/formatter/progress.lua: Color output when --color=no was
not specified.
* specl/formatter/report.lua: Likewise.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>